In my U.S. Pat. No. 4,577,841, dated Mar. 25, 1986, there is shown and described a spring unit wherein there are top and bottom attaching elements connected to each other by a yieldable structure comprising spaced, parallel, vertical legs shorter in length than the distance between the top and bottom attaching elements and at the upper and lower ends inclined legs connecting the upper and lower ends to the attaching elements. Under extreme loads perpendicular to the attaching elements, the vertical legs tend to bow outwardly, which can lead to collapse. It is the purpose of this invention to structurally improve the aforesaid spring unit in such a way as to counteract the tendency for the vertical legs to collapse.